The present invention relates to solid and liquid compositions containing particles of highly fluorinated ion exchange polymer having sulfonate functional groups, a process for making such compositions, and products made using such compositions.
Liquid compositions of perfluorinated ion exchange polymers are known for use in the manufacture and repair of ion exchange membranes, for membrane coatings containing conductive or nonconductive particles, and for many other uses. While such compositions are sometimes referred to as solutions, the compositions are generally recognized as being dispersions of polymer particles.
U.S. Pat. No. 4,433,082 to Grot discloses a process for preparing such liquid compositions containing perfluorinated ion-exchange polymers having sulfonic acid groups or a salt thereof and having equivalent weights of 1025 to 1500. The medium preferred for use in U.S. Pat. No. 4,433,082 contains 20 to 90% by weight of water and 10 to 80% by weight of an organic compound such as a lower alcohol. While U.S. Pat. No. 4,433,082 indicates that liquid compositions can be prepared using water only, no useful processes for making liquid compositions without alcohols are disclosed.
Compositions made in accordance with U.S. Pat. No. 4,433,082 and containing water together with one or more lower alcohols are sold commercially under the trademark NAFION(copyright) by E. I. du Pont de Ne mours and Company. Currently, the commercial compositions can contain up to 10% by weight of a perfluorinated ion-exchange polymer having sulfonic acid groups in a medium of approximately 30-60% by weight water, 15-30% by weight 1-propanol, 15-30% by weight 2-propanol, and less than 10% by weight (total) of miscellaneous components consisting of methanol, mixed ethers and other volatile organic compounds (VOC""s). A typical commercial composition contains a nominal 5% by weight of the perfluorinated ion-exchange polymer having sulfonic acid groups in a medium of approximately 45% by weight water, 22% by weight 1-propanol, 22% by weight 2-propanol, 3% by weight methanol and 3% of mixed ethers and other VOC""s.
For many uses, the alcohol in these compositions is undesirable. For example, known compositions are often used in the manufacture of electrodes containing catalyst particles for electrochemical cells such as fuel cells. Exposure of alcohol vapors to the catalyst particles can cause undesirable side reactions and can even be a fire hazard. In general, the presence of an alcohol means that the alcohol or its decomposition products will be released into the atmosphere when the composition is used. Releases of VOC""s result, not only in the loss of the compounds, but are subject to reporting requirements and limits imposed by environmental authorities. Recovery systems can be employed but they generally require a large investment, are expensive to operate, and may not be cost effective, particularly for small scale operations.
The alcohol in known compositions can be partially or entirely removed to produce a composition which contains less alcohol or essentially only water by processes such as vacuum distillation. However, such compositions are expensive because of the additional processing steps. In addition, the problems relating to alcohol release or recovery are associated with the process used to remove alcohol from the compositions.
Nonaqueous compositions containing alcohol or another organic medium with little or no water content are also desired for some applications. While known compositions containing a mixture of water and alcohol can be converted to alcohol only compositions by processes such as azeotropic distillation, these processes are time consuming and expensive. Nonaqueous compositions in a nonaqueous media other than alcohol have are not typically been available due to the difficulty in manufacture.
Moreover, commercially-available compositions typically have a low concentration of polymer (in the range of about 5% by weight) and are unsuitable for applications where higher concentrations are desired. For example, when coatings are made, it is often necessary to use processes which employ repetitive applications of the composition to make the desired coating thickness and such processes are usually complicated, time consuming and costly.
The present invention provides solid and liquid compositions comprising particles of highly fluorinated ion-exchange polymer having sulfonate functional groups with an ion exchange ratio of less than about 33. At least about 25 weight % of the particles in the composition have a particle size of about 2 nm to about 30 nm. Preferably, the compositions contain at least about 50% by weight, most preferably 90% by weight, of particles having a particle size of about 2 nm to about 30 nm. Preferably, the solid composition is dispersible in water at room temperature and most preferably forms a stable colloid.
In accordance with another aspect of a solid composition in accordance with the invention, the particles have a structure in which the polymer chains are folded so that the fluorine atoms are oriented towards the particle interior and the sulfonate groups are concentrated on the surface. In accordance with preferred form of the invention, sufficient sulfonate groups are on the surface to make the material redispersible in water at room temperature. Moreover, it is preferred for at least 50% of the particles to be monomolecular, i.e., that each particle consists of essentially one polymer molecule. Most preferably, at least 90% of the particles are monomolecular. Preferably, at least about 25% by weight of said particles have a particle size of about 2 nm to about 30 nm.
The liquid compositions in accordance with the invention can contain either an aqueous liquid medium or nonaqueous liquid medium with 0.5 to about 50% by weight of the polymer. The aqueous liquid compositions are preferably substantially free of water miscible alcohols and more preferably are substantially free of all organic liquids. In one preferred form of the invention the medium consists essentially of water. The nonaqueous compositions contain less than about 5% by weight water and preferably the nonaqueous medium comprises a polar, small molecule organic liquid.
Liquid compositions also containing particles having a different composition than the ion exchange particles are also provided by the invention.
The invention also provides a process for preparing an aqueous liquid composition comprising particles of highly fluorinated ion-exchange polymer having sulfonate functional groups and having an ion exchange ratio of less than about 33. The process includes contacting the polymer in a pressurized vessel with an aqueous liquid dispersion medium under conditions which cause the polymer to form particles with at least about 25% by weight of said particles having a particle size of about 2 nm to about 30 nm. The contents of the vessel is cooled to a temperature of less than about 100xc2x0 C. and an aqueous liquid composition comprising particles of the highly fluorinated ion-exchange polymer is recovered. Preferably, the dispersion medium for use in the process is substantially free of water miscible alcohols, the temperature is about 150xc2x0 C. to about 300xc2x0 C., and the contents of the vessel is agitated sufficiently to subject the contents of the vessel to a shear rate of at least about 150 secxe2x88x921.
In one preferred form of the invention, the dispersion medium consists essentially of water.
In another preferred form of the invention, the dispersion medium comprises 0.5 to 75% by weight of a dispersion assist additive selected from the group consisting of nonreactive, substantially water immiscible organic compounds and carbon dioxide.
The aqueous compositions produced in the process can be converted to solid compositions in accordance with the invention by removing liquid components, preferably by evaporation at a temperature less than the coalescence temperature of the ion exchange polymer.
The invention also provides processes for making films and elongated articles such as fibers from highly fluorinated ion exchange polymers and articles containing a substrate coated or impregnated with fluorinated ion exchange polymers.